Sea stars are members of the group of animals that share the characteristic of having a spiny outer covering. The scientific name of this group, or phylum, is Echinodermata (ee-kine’-no-der-mahta), which is formed from the Latin root meaning spiny (echinus-) and the Greek root meaning skin (-derma). All members of the phylum Echinodermata have hard, spiny inclusions in their outer body covering. The most extremely spiny among these animals are the sea urchins which appear as a living pin cushion, while sea cucumbers, by and large, lie at the other end of the spectrum, with many species lacking obvious spines.
Sea stars occupy something of middle ground in the phylum with many having what appears to have simply a very leathery, tough skin, while some, like the crown of thorns sea star have a formidable array of very sharp spines. Sea star species off the coast of California tend to be more of the leathery variety. The armored sand star (Astropecten armatus) is one exception, having obvious spines along the edges of its arms, but these are blunt by comparison to the crown of thorns.
The knobby sea star (Pisaster giganteus) has very noticeable, but knob-like spines scattered all over its upper surface. These can be seen clearly in today’s photo, taken at Santa Rosa Island. The knobby sea star spines typically have a ring of blue around the base of the spine, but in this case the ring is essentially the same color as the spine. Also quite noticeable surrounding each spine are clusters of dermal branchiae, which serve to absorb oxygen from the surrounding water. These dermal “gills” supplement the absorption of oxygen by the soft tissue of the tube feet.
While most knobby sea stars are tan or brown with very obvious blue rings, but, as illustrated by the individual in this photo, they can be found in a yellow/orange color with much more subdued spine colors.
This photo was taken with a Canon EF100 mm f/2.8 Macro USM lens with a Canon EF25 IIand a Canon EF12 II extension tubes on a Canon EOS 5D Mk III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe in eTTL exposure mode. The exposure was set to 1/60 sec at f/9.5 and ISO 200.